全文获取类型
收费全文 | 9718篇 |
免费 | 2085篇 |
国内免费 | 2943篇 |
专业分类
测绘学 | 1192篇 |
大气科学 | 1885篇 |
地球物理 | 2111篇 |
地质学 | 5606篇 |
海洋学 | 1403篇 |
天文学 | 321篇 |
综合类 | 950篇 |
自然地理 | 1278篇 |
出版年
2024年 | 76篇 |
2023年 | 187篇 |
2022年 | 492篇 |
2021年 | 592篇 |
2020年 | 488篇 |
2019年 | 550篇 |
2018年 | 545篇 |
2017年 | 483篇 |
2016年 | 591篇 |
2015年 | 581篇 |
2014年 | 570篇 |
2013年 | 617篇 |
2012年 | 630篇 |
2011年 | 687篇 |
2010年 | 686篇 |
2009年 | 639篇 |
2008年 | 591篇 |
2007年 | 554篇 |
2006年 | 472篇 |
2005年 | 421篇 |
2004年 | 331篇 |
2003年 | 298篇 |
2002年 | 321篇 |
2001年 | 308篇 |
2000年 | 302篇 |
1999年 | 385篇 |
1998年 | 353篇 |
1997年 | 344篇 |
1996年 | 250篇 |
1995年 | 233篇 |
1994年 | 214篇 |
1993年 | 183篇 |
1992年 | 145篇 |
1991年 | 132篇 |
1990年 | 84篇 |
1989年 | 79篇 |
1988年 | 79篇 |
1987年 | 44篇 |
1986年 | 46篇 |
1985年 | 31篇 |
1984年 | 34篇 |
1983年 | 20篇 |
1982年 | 23篇 |
1981年 | 11篇 |
1980年 | 13篇 |
1979年 | 14篇 |
1977年 | 2篇 |
1958年 | 2篇 |
1957年 | 2篇 |
1954年 | 5篇 |
排序方式: 共有10000条查询结果,搜索用时 156 毫秒
921.
Sulfate Attack on Concrete in an Inland Salt Lake Environment 总被引:2,自引:0,他引:2
INTRODUCTIONSulfate attack is one of the reasons for concretestructures deteriorating earlier than usual,thusshowing poor durability.The SO42-from the sur-rounding environment such as ocean,saline,groundwater,and even the concrete itself,reacts with thehydration products of cement,whichleads to the de-terioration of the concrete(Li et al.,2000;Kang,1995;Neville,1983;Samarai,1976).The deteriora-tionincludes three processes:the external swelling,cracking,peeling,and disjointing caused by … 相似文献
922.
西藏改则县托和平错地区展金组中发育一套早—中二叠世以玄武岩类为主、玄武安山岩次之、安山岩少量的海相火山岩组合。该火山岩中玄武岩、玄武安山岩的SiO2含量为44.43%~54.87%,TiO2含量多在4%以上,K2O+Na2O平均为4.57%,玄武岩类富K,玄武安山岩富Na。区内火山岩富集K、Ba、Th、Ta、Ce、P、Zr、Sm、Ti、Cr,亏损Rb、Nb、Hf、Sc,Y/Nb在0.322~0.499之间。综合常量元素和微量元素特征,可将该火山岩归入碱性玄武岩系列。区内火山岩从基性到中性稀土元素总量基本没有变化,REE平均为402.31×10-6,LREE/HREE为8.17~12.69,为轻稀土强烈富集型,δEu为0.93~1.04,平均为0.99,无Eu异常,玄武岩的Th/Ta值多数大于1.6。火山岩的地质特征和地球化学特征表明,本区火山岩产出的大地构造环境为冈瓦纳大陆北缘裂谷环境。 相似文献
923.
924.
925.
926.
927.
928.
929.
Trophic Assessment in Chinese coastal systems-review of methods and application to the Changjiang (Yangtze) Estuary and Jiaozhou Bay 总被引:3,自引:0,他引:3
Yongjin Xiao João G. Ferreira Suzanne B. Bricker João P. Nunes Mingyuan Zhu Xuelei Zhang 《Estuaries and Coasts》2007,30(6):901-918
Coastal eutrophication has become one of the main threats to Chinese coastal areas during the last two decades. High nutrient
loads from human activities have modified the natural background water quality in coastal water bodies, resulting in a range
of undesirable effects. There is a need to assess the eutrophic level in coastal systems and to identify the extent of this
impact to guide development of appropriate management efforts. Traditional Chinese assessment methods are discussed and compared
with other currently-used methods, such as the Oslo-Paris Convention for the Protection of the North Sea (OSPAR) Comprehensive
Procedure and Assessment of Estuarine Trophic Status (ASSETS). The ASSETS method and two Chinese methods were tested on two
Chinese systems: the Changjiang (Yangtze) Estuary and Jiaozhou Bay. ASSETS is process based, and uses a pressure-state-response
model based on three main indices: Influencing Factors, Overall Eutrophic Condition, and Future Outlook. The traditional methods
are based on a nutrient index. ASSETS was successfully applied to both systems, classifying the Changjiang Estuary as Bad
(high eutrophication) and Jiaozhou Bay as High (low eutrophication). The traditional methods led to ambiguous results, particularly
for Jiaozhou Bay, due to the high spatial variability of data and a failure to assess the role of shellfish aquaculture in
nutrient control. An overview of the Chinese coastal zone identifies 50 estuaries and bays that should form part of a national
assessment. A comparison of methods and results suggests that ASSETS is a promising tool for evaluating the eutrophication
status of these systems. 相似文献
930.
Tao Li Zhe Zhu Dongsheng Wang Chonghua Yao Hongxiao Tang 《International Journal of Mineral Processing》2007
Flocs generated by various shear forces exhibit different characteristics of size, strength and structure. These properties were investigated by employing a continuous optical monitoring and a microscope with CCD camera to directly monitor aggregation under six different shear intensities. The floc structure was characterized by the fractal dimension. The results showed that the flocculation index (FI) decreased from 1.16 at 20 rpm to 0.25 at 250 rpm and the floc size decreased from 550 μm to 150 μm, meantime, the FI value showed a good correlation with floc size. In order to determine the floc strength, two methods were used. One was the strength factor, ranging from 18.3% to 62.5%, calculated from FI curve, and the other was a theoretical value between 0.005 N/m2 and 0.240 N/m2, estimated by calculation. The floc strength increased with the G value in both cases. Furthermore, the fractal dimension increased with G and its value was between 1.30 and 1.63. The relation between fractal dimension and strength was also obtained. 相似文献